Work is often carried out in elevated structures, such as high buildings or wind turbines. In the event of an emergency situation, it is necessary to be able to quickly and reliably evacuate workers from the elevated structure. For example, in the event of a serious accident, such as fire, in a wind turbine, any people in the wind turbine may need to self evacuate from elevations of over 100 meters. Since such a self evacuation is often carried out under stress and without any assistance from other personnel, the descending device used for the self evacuation should automatically and reliably keep the descent speed at a suitable level.
To provide for the desired reliability and ease of use, known descending devices typically include a centrifugal brake for limiting the descent speed.
A centrifugal brake typically comprises a fixed part and a rotatable part, where the rotatable part comprises radially movable brake shoes that are pressed towards the fixed part of the centrifugal part through the centrifugal force when the rotatable part rotates. The braking force by which the brake shoes are pressed against the fixed part is proportional to the square of the angular speed of the rotatable part, and proportional to the mass of the brake shoes and the radius of the rotatable part.
In particular in modern wind turbines etc, several persons may be in the elevated structure at the same time. The descending device should therefore be capable of rapid evacuation of several persons, and descending devices requiring rewinding of a rope on a drum between each descent are not favorable.
DE 10 2005 010 767 discloses a descending device for repeated braked descent of a load from an elevated structure, comprising a rope having two different load attachment portions being separated by a predefined descent length of the rope. The rope is partly wound around a descent shaft that is connected to the rotatable part of a centrifugal brake via a gear. A first descent is carried out by attaching the load (such as a person) to descend from the elevated structure to one of the load attachment portions, whereby the descent shaft will be brought to rotate in a first direction of rotation. When the first load has reached the ground, the subsequent descent is carried out by attaching the load to descend from the elevated structure to the other one of the load attachment portions, whereby the descent shaft will be brought to rotate in a second direction of rotation opposite the first direction of rotation.
To ensure that there is no slippage between the rope and the descent shaft, the descending device according to DE 10 2005 010 767 comprises a pulley with a generally V-shaped groove for providing a high friction between the pulley and the rope.
In the descending device according to DE 10 2005 010 767. a gear is provided between the descent shaft and the rotatable part of the centrifugal brake in order to increase the angular speed of the rotatable part of the centrifugal brake such that a sufficient braking force can be achieved for descent at a suitable speed. According to international regulations, descending devices should lower a person at a substantially constant speed of between 0.5 m/s and 2 m/s.
Although the descending device according to DE 10 2005 010 767 provides for repeated braked descent at a substantially constant speed, it would be desirable to provide a descending device with improved reliability.
Furthermore, to allow for manually controlled raising or lowering of a load, the descending device according to DE 10 2005 010 767 further comprises a crank that allows a user to manually rotate the descent shaft. The crank acts on the high-speed side of the gear, i.e. on the side of the rotatable part of the centrifugal brake. To prevent unwanted descent in case the user lets go of the crank, the crank is provided with a catch that can be manually activated depending on the desired direction of rotation. In an emergency situation, a user under stress may, however, use the crank incorrectly. In such cases, the user may, due to the gear ratio, destroy the gear, which may lead to failure of the descending device